The present invention relates to a mechanical quantity measuring apparatus for measuring a mechanical quantity.
What is generally called an RF tag has been developed which uses an electricity supplied through electromagnetic induction to activate a circuit and thereby transmit a preset ID number wirelessly and which is beginning to be applied to a goods distribution management and a management of admission tickets. Attempts are currently under way to connect a physical quantity sensor to such an ID tag. For example, as disclosed in JP-A-2001-187611, a temperature sensor is connected to an RF tag circuit on a printed circuit board, and the temperature sensor mounted on the printed circuit board is then entirely molded with plastic to form an ID tag with a sensor. A sensor whose circuit is formed on a semiconductor substrate is disclosed in JP-A-05-203682 and JP-A-09-264798.
When a strain sensor and a mechanical quantity sensor applying the strain sensor, such as pressure sensor, vibration sensor and acceleration sensor, are connected to a circuit that uses an electricity supplied through electromagnetic induction or microwaves to transmit the result of measurement, however, the following problems characteristic of the mechanical quantity sensor arise. Similarly, when the measurement value of a sensor using a battery or the like as its power source is transmitted or received by electromagnetic waves, the following problems arise. Since various environmental influences are given depending upon usage conditions, a sensor capable of high precision measurement even under such conditions is desired. The above-described well-known techniques do not have concrete disclosure of examples of high precision measurement.
First, the strain sensor has a very small output for a measured strain and is very vulnerable to noises as compared with other sensors such as temperature sensor. For example, in normal use, the strain gauge is required to have a resolution of the order of 10−5 and a resistance variation (ΔR/R) in the most commonly used resistor wire type strain gauge is about 2×10−5. That is, the strain gauge is required to detect when the resistor, whose resistance is 1 in a no-strain condition, produces a resistance of 1.00002 when strained. At this time, if any noises enter the circuit, it may cause large measuring errors. Particularly when the apparatus is operated on an electric power supplied through electromagnetic induction or microwaves, the strain sensor is also subjected to a radio wave, making it easier for noises to enter the circuit. Further, when the electricity supplied by electromagnetic induction or microwaves is used, an amount of electricity that can be supplied to the strain sensor is limited considerably and is required to be set two or more orders of magnitude smaller than when a commonly marketed strain gauge and an amplifier are used. Thus, if the current flowing through the strain sensor is suppressed to 200 μA or smaller, the apparatus becomes susceptible to noises, substantially burying the signal in noises. The strain measurement is often made by directly attaching the sensor to an object being measured. Considering this condition of use, it is difficult to cover the sensor and its lead wires with a conductive material for perfect electromagnetic shield. In usual strain measurement, a strain quantity in a particular direction of an object being measured whose strain generation state is unknown, is required to be measured. A strain sensor is therefore desired which can detect a strain quantity in a particular direction at high precision.
It is therefor an object of the present invention to solve at least one of the above-described problems and provide a mechanical strain measuring apparatus capable of high precision measurement.